1. Technical Field
The present invention relates to an optical scanning device, a control method of an optical scanning device, and an image display apparatus, and more particularly to an improvement of an optical scanning device which provides laser beam scan using laser beams modulated according to image signals for the purpose of image display.
2. Related Art
An image display apparatus which displays images by laser beam scan uses an optical scanning device capable of providing laser beam scan. The optical scanning device executes scan using laser beams modulated according to image signals scan in the two-dimensional direction. The image display apparatus displays images by introducing the laser beams released from the optical scanning device to a screen or the like. For modulating laser beams, pulse width modulation (hereinafter referred to as “PWM”) is generally used. The PWM varies the pulse width during which laser beams are turned on in accordance with image signals. For representing gradations of all pixels in one frame of an image in accordance with image signals, the minimum unit pulse width needs to be extremely small. The minimum unit pulse width further decreases as the number of pixels and the number of gradations in the image increase. However, it is extremely difficult to accurately switch on and off a high-output laser beam source at high speed in accordance with small pulse widths. Thus, when it is difficult to raise modulation frequencies, a plurality of laser beams are used so that scan can be divided among the plural beams modulated according to image signals. For example, in the optical scanning device which provides laser beam scan on the beam-receiving region in the horizontal and vertical directions, scanning can be divided among a plurality of laser beams arranged in parallel in the vertical direction when the frequency at which laser beams scan in the horizontal direction is higher than the frequency at which laser beams scan in the vertical direction. When the plural laser beams are arranged in parallel in the vertical direction, images can be displayed at a lower modulation frequency than in the case where a single laser beam scans. However, large clearances are produced between tracks formed by a group of spots in the beam-receiving region. These clearances become larger as the number of laser beams arranged in parallel increases. Also, scan in the horizontal direction deviates from the horizontal direction of the screen and is obliquely performed. When the clearances between scanning tracks of laser beams become conspicuous, high-quality images cannot be obtained. For providing laser beam scan without generating noticeable clearances, the method has been considered as a solution that the same number of light sources as that of the scanning lines in either the horizontal or vertical direction are equipped to perform laser beam scan only in one direction. JP-A-2003-21804 proposes such a technique using the same number of light sources as that of the scanning lines, for example.
When the same number of light sources as that of the scanning lines are equipped, however, a considerable number of light sources are required and the optical scanning device including these is extremely expensive. As obvious, there has been arising a problem that high quality images are difficult to be displayed when the technique of divided scan using a plurality of beams is used.